A simplified layout of a cellular communications system is depicted in FIG. 1. Mobile stations M1-M10 communicate with the fixed part of a public switched telephone network (PSTN) by transmitting radio signals to, and receiving radio signals from, cellular base stations B1-B10. The cellular base stations B1-10 are, in turn, connected to the PSTN via a Mobile Switching Center (MSC). Each base station B1-B10 transmits signals within a corresponding area, or "cell" C1-C10. Within each cell, a base station transmits to the mobile units over downlink RF channels, while the mobile units transmit information to the base station over uplink RF channels. As depicted in FIG. 1, an idealized arrangement of base stations can be organized so that the cells substantially cover an area in which mobile telephone communication ordinarily occurs (e.g., a metropolitan area), with a minimum amount of overlap.
While cellular systems were originally designed to operate with a one-to-one correspondence between a mobile station and an associated base station covering a geographic cell, it has been determined that the effects of shadowing and fading can be reduced by communicating the same signal to a mobile station over more than one link. For instance, two different base stations can communicate the same information to a mobile station over two different spatially offset links. The mobile station processes the signals from the two links by combining them in some way, e.g., maximal ratio combining. This technique is known as diversity. Conventional spatial diversity techniques employ two or more separated antennas in a single base station, or two or more base stations, to communicate with a mobile station. However, diversity is not limited to spatially offsetting base stations or antennas (i.e., multiple transmission paths). Diversity transmission can be generated using one or more of an offset in time, polarization, or frequency. An example of time diversity is interleaving, which is used in the IS-54B EIA/TIA standard for North American digital cellular systems. Frequency diversity is implemented by transmitting the same information on two different frequencies, however, such a scheme fails to make efficient use of the frequency spectrum. As described briefly above, the concept of space diversity involves the receipt of signals over multiple signal paths.
Because spacial diversity can involve using two entirely different base stations to communicate with a mobile station, the technique has also been called macro diversity. However, as used herein, the term macro diversity can also involve an arrangement wherein antennas used for the diversity transmissions are located close to one another, or even co-located in a same base station. FIGS. 2-5 depict several exemplary macro diversity arrangements.
FIG. 2 illustrates a macro diversity arrangement wherein a first base station 202 and a second base station 204 each transmit a same message 206 to a mobile station 208. The message 206 is transmitted to the mobile station 208 over different signal paths in the forms of a first downlink 210 and a second downlink 212. The first and second downlink signals 210 and 212 are recombined in the mobile station 208 to extract the message 206. The mobile station 208 transmits to the base stations 202 and 204 over first and second uplink paths 214 and 216, respectively.
FIG. 3 illustrates a macro diversity arrangement wherein the same message 306 is broadcast from a first antenna 304 and a second antenna 305. The antennas 304 and 305 have different polarization characteristics, e.g., horizontal and vertical polarization, but are located in the same base station 302. First and second downlinks 310 and 312 communicate the message 306 from the base station 302 to the mobile station 308, while first and second corresponding uplinks 314 and 316 communicate from the mobile station 308 to the base station 302.
FIG. 4 depicts a macro diversity arrangement for an indoor RF communication system in which one or more of first, second and third antennas 402, 404 and 410 transmit a signal containing the same message 406 to a mobile unit 408. As depicted, first and second downlinks 410 and 412 communicate the message 406 from antennas 404 and 410, respectively, to the mobile station 408. First and second uplinks 414 and 416 communicate from the mobile station 408 to the antennas 404 and 410, respectively.
FIG. 5 depicts a single base station macro diversity arrangement wherein first and second directional lobes 518 and 520, generated by an antenna array 504, each cover a separate coverage area. The first directional lobe 518 maintains a first macro diversity link including a first downlink 510 which carries a message 506. The second directional lobe 520 maintains a second macro diversity link including a second downlink 512 which also carries the message 506. First and second uplinks 514 and 516 communicate from the mobile station 508 to the antenna array 504 within each lobe 518 and 520, respectively.
In a macro diversity arrangement, the base stations and/or antennas communicating with a particular mobile station are known as "active set" members. For example, referring back to FIG. 4, antennas 404 and 410 would be considered members of the active set. Members of an active set can change as the mobile station passes into and out of coverage areas handled by base stations and/or antennas in the system. As known to those skilled in the art, the addition and deletion of base stations and/or antennas to and from an active set can be used to achieve handoff.
Macro diversity increases robustness, achieves improved downlink quality, and combats fading. However, the additional active transmitting elements in a macro diversity scheme increases interference (i.e., the C/I ratio) for surrounding mobile and/or base stations operating in the vicinity. Conventional macro diversity systems ordinarily utilize the same amount of downlink transmit power for each antenna in the active set. For instance, in IS-95 systems, the same transmit power level is used for all downlinks in the active set. Because of the undesirable interference to other users, careful consideration is required in adding and deleting members from the active set so that the interference in unrelated links is minimized. Accordingly, one method for controlling interference is to limit the number of base stations and/or antennas in an active set.
Another method used in conventional systems to reduce the effects of unnecessary interference from macro diversity operation is power split control. In power split control, the downlink transmit power may be equally split between each active base station and/or antenna in an active set. That is, in the case where there are three downlinks, and a total transmission power of P is available, each of the downlinks has a transmission power level of P/3. However, even with such an allocation, there may be an unnecessary amount of interference introduced when the "weakest" downlink in the active set is operated at a P/3 power level. More specifically, the link may, in effect provide a small improvement in communications robustness, but introduce, on balance, a greater amount of disruption to surrounding communications by unduly introducing interference. Consequently, the C/I ratio for adjacent cells can be negatively impacted with only a minimal gain in communications efficiency.
In DS-CDMA systems, a proportional downlink transmission power control method is used that allots downlink transmit power in accordance with the characteristics of the downlink signal received by mobile stations. More specifically, an amount of transmit power used for a downlink is defined based on pilot channel signal strength and interference values of downlink signals as measured at the mobile station. The measurement information is then reported to the system by the mobile station. Such a system is desirable because it permits only a minimum amount of power to be used to maintain a desirable level of communication efficiency, while at the same time introducing a minimal amount of interference to adjacent, unrelated, links. However, the DS-CDMA system has many shortcomings.
For example, DS-CDMA systems require significant information overhead and consume important resources to carry out downlink power control. The DS-CDMA downlink power control system uses mobile stations to periodically measure path gain characteristics for cells adjacent to the mobile station. Measurement reports are regularly transmitted reported back on associated uplinks. Because so-called "fast" power control is frequently used in DS-CDMA systems, the measure and transfer of information can require that approximately 10% of uplink frame capacity be dedicated to downlink measurement information and reports. Consequently, there is less frame capacity for other information. The mobile station must also perform additional processing to measure, format and transmit the measurement information. This has the effect of consuming processing resources, elevating design complexity and increasing handset power consumption.
What is needed is a system of downlink transmit power control for each antenna and/or base station downlink which does not diminish the information capacity of frames transmitted by a mobile station. It would be further desirable to provide a downlink power control system that does not burden the mobile station with additional tasks such as measuring the quality of a downlink, and processing and transmitting the measurement data.